The present invention relates to a rustproof steel sheet for automobile fuel tanks which has excellent resistance weldability, corrosion resistance and press formability. The invention further relates to an automobile fuel tank with excellent corrosion resistance and to a seam welding process for automobile fuel tanks.
Automobile fuel tanks usually have a final design which is in conformity with the design of automobile bodies, and their shapes therefore tend to be very complicated. Their structure includes, as shown in FIG. 1, a fuel supply opening 3, a fuel supply pump (not shown), a fuel hose 4, the fuel hose 4 serving to return excess fuel 6, separators 5 to prevent the sound of fuel waves, etc. The fuel tank body 1 consists of a pair of bowl-shaped molds formed into an integral whole by seam-welding the flange members 2. Each of the parts are bonded by spot welding, soldering or brazing.
This fuel tank is an important member of the automobile from a safety standpoint, and it is required to possess the features of sufficient corrosion resistance against fuel, leakproofness and impermeability to fuel, and also low fatigue after forming and resistance to cracking by impacts. The corrosion resistance is of course to eliminate the concern of corrosion holes, but it is also important in terms of preventing production of abundant corrosion products which lead to clogging of the filter at the inlet of the fuel pump in the fuel tank.
Various modifications have been made in the materials, manufacture and manufacturing processes to obtain fuel tanks with such properties. As a result of modifications in the materials, it has become common to employ fuel tanks made of Pbxe2x80x94Sn plated steel sheets which have sufficient corrosion resistance against fuel, low generation of corrosion products and easier welding and soldering suitability for better production efficiency (Japanese Examined Patent Publication No. 57-61833). However, Pb is a metal which is detrimental to the environment, as is well known. Also, while Pbxe2x80x94Sn plated steel sheets are well suited for soldering and brazing as mentioned above, the soldering component is an Snxe2x80x94Pn system which of course contains Pb. Consequently, with recent demands for fuel tanks which employ absolutely no Pb, fuel tanks made of Alxe2x80x94Si based alloy plated (hereunder, xe2x80x9caluminizedxe2x80x9d) steel sheets have become a focus of study as candidate substitutes.
Aluminized steel sheets are one type of material which utilize no Pb and have satisfactory corrosion resistance and workability. Aluminizing forms a stable oxide film on the surface, and therefore provides satisfactory corrosion resistance against not only gasoline but also alcohol and organic acids produced by degradation of gasoline. However, several problems arise when aluminized steel sheets are used as fuel tank materials. One of these is workability, and aluminized steel sheets (especially hot dip aluminized steel sheets) are susceptible to plating layer peel and plating layer cracks originating from sections of very hard Fexe2x80x94Alxe2x80x94Si intermetallic compounds (hereunder referred to as the xe2x80x9calloy layerxe2x80x9d) produced at the interface between the coated layer and the steel sheet. The present inventors have dealt with this issue in Japanese Patent Application No. 7-329193, disclosing that it can be overcome by adjusting the cooling rate and reheating after plating. Another problem is weldability. Specifically, although aluminized steel sheets are suitable for resistance welding such as spot welding and seam welding, the coated Al metal has high affinity for Cu which is usually used as the electrode, and forms brittle Alxe2x80x94Cu or Alxe2x80x94Cuxe2x80x94Fe alloys on the electrode surface during welding, thus resulting in the problem of gradual loss during continuous operation and early welding defects.
Aluminized steel sheets have been conventionally used after being subjected to chromate treatment, mainly with chromic acid and silica, for the purpose of improving corrosion resistance, and disclosed instances thereof include Japanese Examined Patent Publication No. 4-68399, Japanese Unexamined Patent Publication No. 58-6976, Japanese Unexamined Patent Publication No. 58-48679 and Japanese Unexamined Patent Publication No. 60-56072. All of these methods, however, contribute little to improvement in continuous operation because the reactions with the electrode are virtually the same as with untreated materials. The process of Japanese Examined Patent Publication No. 4-68399 is characterized by forming the coating to 35-70 mg/m2 in terms of Cr, but although corrosion resistance of the fuel tank is achieved with this amount of coating, there is a disadvantage for spot welding and seam welding, in that the Al in the plating layer tends to form alloys with the electrode Cu as with untreated materials, so that the electrode tip becomes alloyed during continuous operation thus shortening the life of the electrode. In addition, if the brazing material is not carefully selected, the wettability of the brazing material will be lower resulting in the problem of a more difficult brazing operation, and tanks with brazed pipes, etc. will be difficult to manufacture. Japanese Unexamined Patent Publications No. 58-6976 and No. 58-48679 disclose processes characterized by the amount of chromate coating of 5-40 mg/m2 in terms of Cr and organic silicon water repellent treatment, but in addition to the same problems with resistance welding as Japanese Examined Patent Publication No. 4-68399, the corrosion resistance for fuel tanks is poor at less than 10 mg/M2 even with organic silicon water repellents, and the corrosion resistance against organic acids produced by degradation of gasoline fuel is insufficient. Also, as in Japanese Examined Patent Publication No. 4-68399, despite the improved corrosion resistance at 35 mg/m2 and greater, failure to carefully select the brazing material will result in a lower wettability of the brazing material, thus complicating the brazing operation. Another disadvantage is that in spot welding and seam welding, the Al in the plating layer tends to form alloys with the electrode Cu as with untreated materials, so that the electrode tip becomes alloyed during continuous operation thus shortening the life of the electrode. The process in Japanese Unexamined Patent Publication No. 60-56072 is characterized by the amount of chromate coating of less than 10 mg/m2, and thus its drawback is that it cannot provide the weldability or the corrosion resistance required for fuel tanks. With these conventional techniques, it has been difficult to satisfactorily achieve the resistance weldability, continuous operation and corrosion resistance required for production of fuel tanks.
It is an object of the present invention to provide an aluminized steel sheet for a fuel tank material, which improves resistance weldability over rustproof steel sheets for fuel tanks for which conventional aluminized steel sheets have not been suitable, as well as satisfactory press formability and corrosion resistance.
It is another object of the invention to provide a novel fuel tank which is environmentally friendly by not using Pb, and which has excellent corrosion resistance in environments of gasoline and other fuels.
It is yet another object of the invention to provide a seam welding process by which it is possible to achieve improved resistance weldability over rustproof steel sheets for fuel tanks for which conventional aluminized steel sheets have not been suitable, as well as continuous operation.
The present invention provides the following in order to attain the objects described above.
(1) A coating aluminized steel sheet suitable for fuel tanks, which comprises
(a) a steel sheet,
(b) an aluminizing layer formed on one or both sides of the steel sheet and based on aluminum or an aluminum alloy containing 2-15 wt % silicon, and
(c) a coating layer formed on at least one of the aluminizing layers and selected from the group consisting of
i) an organic and inorganic composite chromate film having a film thickness of 0.1-2 xcexcm and containing a resin and a chromic acid compound, with the resin/metal chromium weight ratio in the range of 0.5-18,
ii) an inorganic-based chromate film A with the coating layer formed to 10-200 mg/m2 in terms of metallic chromium, which comprises 100 parts by weight of chromic acid in terms of metallic chromium and 100-1000 parts by weight of colloidal silica, and further comprises at least one selected from the group consisting of 100-600 parts by weight of a phosphoric acid compound, 10-200 parts by weight of a phosphonic acid or phosphonic acid salt compound and less than 50 parts by weight of an organic resin, and
iii) an inorganic-based chromate film B with a coating amount of at least 10 mg/m2 and less than 35 mg/m2 in terms of metallic chromium.
(2) A coating aluminized steel sheet according to (1) above, wherein the aluminizing layer is formed to 60 g/m2 or less.
(3) A coating aluminized steel sheet according to (1) or (2) above, wherein the composite chromate film further contains 0.5-20 wt % of a lubricant.
(4) A coating aluminized steel sheet according to (1), (2) or (3) above, wherein the composite chromate film further contains 100-600 parts by weight of a phosphoric acid compound and 100-1000 parts by weight of colloidal silica with respect to 100 parts by weight of metallic chromium.
(5) A coating aluminized steel sheet according to (4) above, wherein the composite chromate film further contains 10-200 parts by weight of a phosphonic acid or phosphonic acid salt compound with respect to 100 parts by weight of metallic chromium.
(6) A coating aluminized steel sheet according to any of (1) to (5) above, which has the aluminizing layer on both sides of the steel sheet and which has the composite chromate film on the aluminizing layers on both sides.
(7) A coating aluminized steel sheet according to (1) or (2) above, which has the aluminizing layer on both sides of the steel sheet, and which has the inorganic-based chromate film A) on the aluminizing layers on both sides.
(8) A coated aluminized steel sheet according to any of (1) to (5) above, which has the aluminizing layer on both sides of the steel sheet and which has the composite chromate film on the aluminizing layer on one side and an inorganic-based chromate film C with a coating amount of 200 mg/m2 or less in terms of metallic chromium on the aluminizing layer on the other side.
(9) A coating aluminized steel sheet according to (8) above, wherein the inorganic-based chromate film C formed on the aluminizing layer further contains at least one selected from the group consisting of phosphoric acid compounds, phosphonic acid and phosphonic acid salt compounds, and less than 50 parts by weight of a resin with respect to 100 parts by weight of metallic chromium.
(10) A coating aluminized steel sheet according to (1) or (8) above, which has an inorganic-based chromate film C at 100 mg/m2 or less in terms of metallic chromium between the aluminizing layer and the composite chromate film.
(11) A coating aluminized steel sheet according to (10), wherein the inorganic-based chromate film C formed between the aluminizing layer and the composite chromate film further contains at least one selected from the group consisting of phosphoric acid compounds, phosphonic acid and phosphonic acid salt compounds, and less than 10 parts by weight of a resin with respect to 100 parts by weight of metallic chromium.
(12) A coating aluminized steel sheet according to (1) above, which has the aluminizing layer on both sides of the steel sheet, and which has the inorganic-based chromate film B formed to 10-35 mg/m2 in terms of metallic chromium on the aluminizing layers on both sides.
(13) A coating aluminized steel sheet according to any of (1) to (5) above, which has the aluminizing layer on both sides of the steel sheet, and which has the composite chromate film on the aluminizing layer on one side and an inorganic resin film with a thickness of 0.1-2.0 xcexcm on the aluminizing layer on the other side.
(14) A coating aluminized steel sheet according to (13) above, which has an inorganic-based chromate film C at 100 mg/m2 or less in terms of metallic chromium between the aluminizing layer and the composite chromate film and/or the organic resin film.
(15) A coating aluminized steel sheet according to (14), wherein the inorganic-based chromate film C formed on the aluminizing layer further contains at least one selected from the group consisting of phosphoric acid compounds, phosphonic acid and phosphonic acid salt compounds, and less than 50 parts by weight of a resin with respect to 100 parts by weight of metallic chromium.
(16) A coating aluminized steel sheet according to (1) above, which has the aluminizing layer on both sides of the steel sheet and which has the inorganic-based chromate film B) on the aluminizing layer on one side and an organic-based resin film on the aluminizing layer on the other side.
(17) A coating aluminized steel sheet according to (16), wherein the inorganic-based chromate film B) is formed to 200 mg/m2 in terms of metallic chromium.
(18) A coating aluminized steel sheet according to (17), wherein the inorganic-based chromate film B formed on the aluminizing layer further contains at least one selected from the group consisting of phosphoric acid compounds, phosphonic acid and phosphonic acid salt compounds, and less than 50 parts by weight of a resin with respect to 100 parts by weight of metallic chromium.
(19) A coating aluminized steel sheet according to (17) above, which has an inorganic-based chromate film C at 100 mg/m2 or less in terms of metallic chromium between the aluminizing layer and the organic resin film.
(20) A coating aluminized steel sheet according to (19), wherein the inorganic-based chromate film C formed between the aluminizing layer and the organic resin film further contains at least one selected from the group consisting of phosphoric acid compounds, phosphonic acid and phosphonic acid salt compounds, and less than 5 parts by weight of a resin with respect to 100 parts by weight of metallic chromium.
(21) A fuel tank produced with a coating aluminized steel sheet according to any of (1) to (20) above.
According to the invention, the following are particularly provided as automobile fuel tanks with excellent corrosion resistance.
(22) An automobile fuel tank wherein a pair of bowl-shaped bodies with flanges are integrated by continuous seam-welding of the flange substances, the automobile fuel tank being characterized in that the materials of which the bowl-shaped bodies are made are coating aluminized steel sheets which consist of aluminized steel sheets each having on one or both sides an aluminizing layer based on aluminum or an aluminum alloy containing 2-13 wt % silicon, and having a resin coating on the uppermost surface of the inner and/or outer side.
(23) An automobile fuel tank according to (22) above, wherein the resin coating is an organic and inorganic composite chromate film consisting of a mixture of a resin and a chromic acid compound.
(24) An automobile fuel tank according to (22) above, wherein the resin coating has a thickness of 0.1-2 xcexcm.
(25) An automobile fuel tank according to (22) above, wherein the coating aluminized steel sheets are coating aluminized steel sheets according to any one of (1) to (20) above.
The present invention still further provides the following as seam welding processes for fuel tanks.
(26) A seam welding process for fuel tanks, in which two coating aluminized steel sheets are combined which are aluminized steel sheets each having formed on one or both sides an aluminizing layer based on aluminum or an aluminum alloy containing 2-13 wt % silicon and having a resin coating formed on the one or both sides thereof, wherein the coating aluminized steel sheets have their aluminizing layer at least on the side corresponding to the inner side of the fuel tank, a resin film is provided on at least one of the steel sheet surfaces at the side where the steel sheets meet and/or on at least one of the steel sheet surfaces at the side where it will contact with an electrode wheel, and the two combined steel sheets are then seam welded between a pair of electrode wheels.
(27) The process according to (26) above, wherein the resin film contains a chromic acid compound at 10-200 mg/m2 in terms of metallic chromium.
(28) The process according to (27) above, wherein the resin film has a thickness of 0.1-2 xcexcm.
(29) The process according to (26) above, wherein the resin film formed on the surface of the aluminized steel sheet is an organic and inorganic composite chromate film according to (1) above.
(30) The process according to (26) above, wherein the coating aluminized steel sheet is a coating aluminized steel sheet according to any one of (1) to (11) or (13)-(20) above.